A federal judge temporarily blocked government approval of Molecular Biosystems Inc.'s newest heart-imaging agent. U.S. District Judge Paul L. Friedman ordered the Food and Drug Administration not to approve the product, known as FS069, until it reconsiders a challenge by rival companies that say the FDA is using the wrong classification system to evaluate the ultrasound imaging agent. At stake are U.S. sales that could reach $150 million a year for the product, according to analysts. San Diego-based MBI, with net sales of only $3.1 million last year, has staked its future on FS069. The ruling was a victory for DuPont Merck Pharmaceutical Co., Sonus Pharmaceuticals Inc. and Bracco Diagnostics Inc., which want the FDA to consider FS069 under more rigorous drug-testing standards, rather than as a medical device. Share prices of San Diego-based MBI have fallen 40% since Feb. 24 amid concern that MBI's rivals would try to block final approval of FS069. Shares of the company fell 37.5 cents to close at $8 on the NYSE. Friedman issued his opinion after the market closed.